From 0:30 to 1:10, you have Elixir of Vile and 7/7 mana. You play Forbidden Shaping, it costs you 7 health to summon a 7-cost minion. Then you target Sea Reaver with Forbidden Words, it also costs you 7 health.
And if your health is not enough to pay the cost, you will not take any damage. From 2:00 to 2:20, you have only 8 health and 10/10 mana. Forbidden Shaping summons Varian Wrynn but it costs you no health or mana. Forbidden Words also costs you nothing.
The reason this bug is not noticed in normal mode is that Bloodbloom/Cho'Gall is a one-time effect. It is consumed when you play the forbidden spell (in on-play phase). In special situations, this can also be done in normal mode: Dead Man's Hand shuffle Forbidden spell into deck, play Cho'Gall, and at the end of turn Grand Archivist plays the Forbidden spell.
https://www.bilibili.com/video/av54205325
From 0:30 to 1:10, you have Elixir of Vile and 7/7 mana. You play Forbidden Shaping, it costs you 7 health to summon a 7-cost minion. Then you target Sea Reaver with Forbidden Words, it also costs you 7 health.
And if your health is not enough to pay the cost, you will not take any damage. From 2:00 to 2:20, you have only 8 health and 10/10 mana. Forbidden Shaping summons Varian Wrynn but it costs you no health or mana. Forbidden Words also costs you nothing.
The reason this bug is not noticed in normal mode is that Bloodbloom/Cho'Gall is a one-time effect. It is consumed when you play the forbidden spell (in on-play phase). In special situations, this can also be done in normal mode: Dead Man's Hand shuffle Forbidden spell into deck, play Cho'Gall, and at the end of turn Grand Archivist plays the Forbidden spell.